The invention relates to a process for producing a substrate with at least one layer of aluminum nitride ceramic, in which one auxiliary or intermediate layer of aluminum oxide is applied to at least one side of this layer.
The invention further relates to a substrate with at least one layer of aluminum nitride (AlN) which is provided on at least one surface side with an intermediate or auxiliary layer which contains aluminum oxide (Al.sub.2 O.sub.3) and which has a thickness in the range of roughly 0.5-10 microns.
Ceramic substrates for electrical circuits or modules, especially for power circuits or modules, are known in the most varied versions. In particular, it is known to produce a metal coating, required for making printed conductors, terminals, etc., on an aluminum oxide ceramic using the so-called "DCB process" (direct copper bond technology) and using copper foils which are oxidized on their surfaces and which form the metallic coating. The copper oxide layer of these foils forms a eutectic with a melting point below the melting point of the copper so that by placing the foils on the ceramic and by heating all the layers they can be joined to one another, especially by melting on the copper, only in the area of the oxide layer. The DCB process is a technique known to one skilled in the art.
In power circuits, the use of an aluminum nitride ceramic instead of an aluminum oxide ceramic is desirable due to the thermal conductivity of the aluminum nitride ceramic, which is higher than that of the aluminum oxide ceramic.
In this case, the DCB process cannot be easily used for aluminum nitride ceramics.
It has been suggested to first apply a layer of aluminum oxide to the layer of aluminum nitride ceramic, such that this intermediate or auxiliary layer enables the application of a metallic coating or the copper layer using the DCB process. This intermediate layer has a roughness which improves adhesion on its exposed surface (DE-OS 35 34 886).
Furthermore, it was proposed that an aluminum oxide layer be applied to an initial substrate or carrier body of aluminum nitride, by flame spraying an aluminum oxide powder or in a screen printing process (DE-OS 38 44 264).
In all these known DCB processes, the disadvantage is that in spite of applying the auxiliary or intermediate layer of aluminum oxide, a flawless homogenous flat bond without faults is not achieved between the layer of aluminum nitride ceramic and the metal coating, but rather numerous faults occur, i.e., areas in which no bond has been formed or the metal coating has been lifted away from the ceramic by bubble formation. Thus the adhesive strength of the metal coatings and the thermal conductivity of the substrates overall are adversely affected.
To improve adhesive strength or bond quality, i.e., to reduce the bubble portion, it is proposed in DE 41 04 860.1 that oxidation of the aluminum nitride (AlN) be done to form the intermediate layer of aluminum oxide (Al.sub.2 O.sub.3). This is accomplished in an oxygen atmosphere free of water vapor.
Conversely, it is proposed in WO 92/11 113 that the aluminum nitride be oxidized and cooled in a controlled manner under a water vapor-containing atmosphere to form the intermediate layer of aluminum oxide.
In JP 02-124 773 A (in Patent Abstracts of Japan C-743, Vol. 14/No. 342) it is proposed that the adhesive strength of the metal coating be increased by incorporating CaO and SiO.sub.2 into the aluminum oxide (Al.sub.2 O.sub.3) layer, CaO and SiO.sub.2 originating from the sintering aids of the aluminum nitride (AlN) body.
One disadvantage of the aforementioned processes, is that the process parameters must be very accurately maintained and further these processes are not applicable to highly heat-conductive aluminum nitride which contains yttrium oxide (Y.sub.2 O.sub.3) as the sintering aid.
In JP 03-228 885 A (in Patent Abstracts of Japan C-890, Vol. 16/No. 1) it is proposed in order to increase the adhesive strength that the aluminum oxide layers be doped with one or more elements of the group Ti, V, Mo, Mb, W, Co or Ni.
In WO 92/11 113 reference is made to a publication of Kuromitsu which relates to SiO.sub.2 --Al.sub.2 O.sub.3 intermediate layers for increasing adhesive strength.
In our own extensive studies, however, the described results of the aforementioned known processes could not be confirmed. It is thus apparent that the process parameters must be precisely controlled or negative results occur even with small deviations.
The incorporation of SiO.sub.2 cited in WO 92/11 113 moreover also engenders the danger that SiO.sub.2 is present not as SiO.sub.2 bound as mullite, but as free SiO.sub.2 which reacts at the eutectic temperature with copper oxide to form a liquid phase and consumes the Cu--Cu.sub.2 O eutectic which is necessary for the DCB process, with which the adhesive strength or bond quality would be strongly reduced.
The object of the invention is to devise a process for producing a ceramic substrate which avoids the aforementioned disadvantages and makes it possible to apply the metal coating to an aluminum oxide ceramic over a large area without faults.